// #pragma GCC push_options
// #pragma GCC optimize ("-O3")

    int  counter = 0; 
    int* counter_ptr  = &counter;

    void setup() {
      cli();                      //stop interrupts for till we make the settings
      /*1. First we reset the control register to amke sure we start with everything disabled.*/
      TCCR1A = 0;                 // Reset entire TCCR1A to 0 
      TCCR1B = 0;                 // Reset entire TCCR1B to 0
      /*2. We set the prescalar to the desired value by changing the CS10 CS12 and CS12 bits. */  
      TCCR1B |= B00000100;        //Set CS12 to 1 so we get prescalar 256  
      /*3. We enable compare match mode on register A*/
      TIMSK1 |= B00000010;        //Set OCIE1A to 1 so we enable compare match A 
      /*4. Set the value of register A to 62500*/
      OCR1A = 62500;              //Finally we set compare register A to this value  
      sei();                      //Enable back the interrupts
        
      Serial.begin(115200);
    }

    void loop() {
      // put your main code here, to run repeatedly:
      Serial.println(counter);
    }

    //With the settings above, this IRS will trigger each 1s.
    ISR(TIMER1_COMPA_vect){
      TCNT1  = 0;                  //First, set the timer back to 0 so it resets for next interrupt

      *counter_ptr = *counter_ptr + 1;
    }
// #pragma GCC pop_options